This invention relates to corrosion inhibiting organic polymer-containing quenchant compositions that provide excellent protection against the corrosion of steel.
Quenching is a process whereby a metal heated to a given elevated temperature is rapidly cooled by immersion in a bath containing a composition having a high heat extracting potential such as water, brines, oils or polymer solutions. Water and brine baths are easily disposed of and relatively inexpensive, however, such baths cool at extremely rapid rates and frequently provide metals quenched therein with a strained microstructure that is susceptible to warpage and cracking. Oil baths typically provide metals quenched therein with relatively slow cooling rates, however, oils are expensive materials to use, have relatively low flash points which create a risk of fire, and oftentimes leave an undesirable film on metals quenched therein.
Low cost aqueous solutions or dispersions of organic polymers have been developed which combine many of the cooling rate advantages of oils with the safety and disposal features of water and brine baths. Unlike oils which tend to form undesirable degradation products which are removed from tanks prior to bath replacement, organic polymer-containing quench baths generally do not form system-fouling products. Thus, these organic polymer-containing compositions are of particular interest. Accordingly, organic polymer-containing baths are easily maintained by the addition of quenchant concentrate to an existing bath.
Aqueous organic polymer-containing quenchant compositions, like any water containing compositions, have a tendency to corrode metals with which they are in contact. Moreover, at the elevated temperatures encountered when hot metals are rapidly cooled, many organic polymers tend to undergo thermal and oxidative degradation to form products, usually acids, which also contribute to corrosion. In order to impart protection against corrosion, one or more corrosion inhibitors are generally included in quenchant compositions. The ability of a corrosion inhibiter to protect against the corrosion of ferrous metals such as iron and steel is especially critical in the case of these compositions since quench tanks, quenchant agitation equipment and workpiece handling systems are typically fabricated from iron based metals.
Under conditions of normal usage, a quench bath gradually becomes depleted of its various components. Bath depletion may result, for example, from evaporation, boil over, and splash out loss of quenchant; polymer being dragged out of solution as a film on materials quenched; and oxidative and thermal degradation of polymer.
In the course of use, the fluid level of the bath is maintained at a predetermined volume by the addition of water, however, since polymer and inhibitor components may be selectively lost from a bath, the periodic addition of concentrated quenchant is necessary to maintain proper bath composition. Users are generally able to readily determine the polymer concentration of a bath by measuring the viscosity and/or refractive index of a bath sample. Analysis of a bath for its inhibitor concentration is typically not performed since it requires more expensive equipment and is more time consuming than polymer analysis.
Since concentrate addition is usually determined by polymer analysis, quenchant compositions are typically formulated to contain an amount of inhibitor in excess of the minimum amount needed to provide protection against corrosion. The safety margin afforded by the inhibitor excess ensures that adequate corrosion protection is not unknowingly lost.
Heretofore, alkali metal nitrites, such as sodium nitrite, have been widely used as inhibitor additives in quenchant compositions. Nitrites are particularly effective in inhibiting the corrosion of steel and from a cost perspective are relatively inexpensive. Amines, such as triethanolamines, are also well know corrosion inhibitors. While not normally used in conjunction with amines, nitrite inhibited quenchant composition are sometimes subject to amine contamination. The formation of N-nitrosamines by the reaction of secondary amines with nitrites may be possible. Therefore, considerable interest exists in developing a non-nitrite corrosion inhibitor system, most preferably an amine-free, non-nitrite corrosion inhibitor system, which may be compatibly added to existing nitrite containing baths without engendering N-nitrosamine formation, thus allowing users to run nitrite out of their baths while converting to non-nitrite systems.
Researchers have experimented with a variety of compositions in attempting to develop a non-nitrite inhibitor system which approximates nitrite performance with respect to inhibiting the corrosion of steel. European Patent Application No. 79236 discloses a nitrite-free quench bath comprising water, a polyalkylene glycol, triethanolamine, salicylic acid and a copper chelating agent. West German Pat. No. 1,811,591 discloses a quench bath comprising water, an ethanol amine, and a carboxylic acid such as benzoic or salicylic acid. Japanese Pat. No. 57-85923 discloses aqueous polyalkylene glycol metal-quenching fluids containing a mixture of a carboxylic acid, an amine and a chelating agent as a corrosion inhibitor.
U.S. Pat. No. 4,263,167 to B. Mago discloses poly(alkylene) oxide compositions suitable for use as hydraulic fluids, metal working lubricants, metal treating formulations, and the like. The compositions have incorporated therein a bridged dimer of a hydroxyl-substituted aromatic carboxylic acid and salts thereof of the general formula: ##STR1## wherein X is a chemically stable group selected from lower alkylene, sulfonyl, and amino groups, and a sulfur atom, and Y and Y' may be the same or different, and are a hydrogen atom, hydroxyl group, amino group, alkyl group, or sulfonyl group.
U.S. Pat. No. 4,277,366 to B. Mago describes poly(alkylene oxide) compositions suitable for use as hydraulic fluids, metal working lubricants, metal treating formulations and the like having incorporated therein an effective amount of a mononuclear aromatic compound having at least one substituent nitro group such as nitrobenzoic acid, 4-nitrophthalic acid, 5-nitroisophthalic acid, 3,5-dinitrobenzoic acid and salts thereof. These compounds are excellent corrosion inhibitors but have the disadvantage of being relatively expensive.
European Pat. No. 0 039 488 issued to Mago et al. discloses poly(alkylene oxide) compositions suitable for use as hydraulic fluids, metal working, lubricants, metal treating formulations and the like having incorporated therein a corrosion inhibiting amount of a mixture comprising a mononuclear aromatic compound having at least one substituent nitro group, a precipitant anodic inhibitor, and preferably a buffer. Included among the precipitant anodic inhibitors suitable for use therein are bridged dimers of hydroxyl substituted aromatic carboxylic acids and salts thereof of the general formula: ##STR2## wherein n is an integer having a value from 1 to 4 and M is hydrogen or an alkali metal; non-oxidizing inorganic acids such as those of tungstic and molybdic acids; and benzoic acid or the soluble salts thereof.
Among the buffers listed as suitable for use therein are those buffers having a pH between about 8 and 10 such as borax and alkanolamines. The patent exemplifies as a suitable inhibitor system the combination of monoethanolamine, benzoic acid, and the sodium salt of m-nitrobenzoic acid (see Table I, Example 9).
The European patent to Mago et al. does not specifically disclose the use of salicyclic acid or the alkali metal salts thereof in combination with nitro-substituted monoaromatics. In fact, in U.S. Pat. No. 4,277,366, Mago discloses that salicylic acid salts and benzoic acid salts behave in different ways, and further, that salicylic acid salts are not effective corrosion inhibitors:
"The data thus presented show that nitroaromatic compounds exhibit considerable oxidation protection for the poly(alkylene oxide) solution and when suitable amounts of said additive were employed, offered good corrosion protection to steel. In contrast thereto sodium salicylate, which has a hydroxyl substituent on an aromatic acid nucleus, exhibited good oxidation protection but was not a good corrosion inhibitor for steel; sodium benzoate, which has no substituent on an aromatic acid nucleus, exhibited good corrosion inhibition for steel but afforded poor oxidation resistance . . . " (see column 5, line 47 to column 6, line 43).
U.S. Pat. No. 4,434,066 to Lewis discloses an aqueous composition suitable for use as a hydraulic energy transmitting fluid having a viscosity of at least 10 centistokes at 40.degree. C. containing up to 80 percent by weight of water and having incorporated therein at least 0.1 percent of an acidic lubricating agent and an amount of an anti-wear additive which is effective in enhancing the antiwear and lubricating characteristics of the composition. The antiwear additive disclosed is a combination of nitroaromatic compounds having at least one substituent nitro group, such as 3-nitrobenzoic acid, 3,5-dinitrobenzoic acid and the like, and a hydroxyl substituted aromatic carboxylic acid, such as salicylic acid, dihydroxy substituted benzoic acid and bridged dimers of the formula: ##STR3## wherein M is hydrogen, alkali metal, alkaline earth metal or ammonium and n is an integer having a value of 1 to 4.
The patent further discloses that a water-soluble polymer viscosity control agent, such as a poly(alkylene oxide) polymer, polyalkyl methacrylate, urethane polymer, polyamide ester, or polyamide alkoxylate may also be present. Lewis specifically discloses the desirability of adding corrosion inhibitors to the composition in addition to the antiwear additive. Examples of corrosion inhibitors suitable for use in the composition are alkanolamines, alkylene oxide adducts, alkali metal salts of carboxylic acids and the like.
In accordance with the present invention there are provided aqueous polymer compositions suitable for use as quenchant compositions which provide excellent protection against the corrosion of metals, particularly steel, having incorporated therein non-nitrite inhibitor systems wherein the corrosion protection afforded by the corrosion inhibitor components of the systems is enhanced by the presence of potentiators. Further aspects of this invention are directed to inhibitor systems that are free from both amine and nitrite components and can thus be used in conjunction with quenchant concentrates for use in quenchant baths that contain nitrites. The invention is also directed to a method of quenching metals using the quenchant composition of this invention.